Calibrator for underwater hydrophones



E. PODOLAK ETAL CALIBRATOR FOR UNDERWATER HYDROPHONES Dec. 22, 1959 2 Sheet-S heet 1 Filed Oct. 27, 1954 RECORDER AUDIQ OSCILLATOR AMPLIFIER F l G.

INVENTORS EDWARD PODOLAK MURRAY ST'RASBERG I Dec. 22, 1959 E. PODOLAK EIAL 2,918,651

CALIBRATOR FOR UNDERWATER HYDROPHONES Filed Oct. 27, 1954 I 2 Sheets-Sheet 2 38 INVENTORS EDWARD PODOLAK MURRAY STRASBERG v I BY 1..

ATTORNEYS sensitivitybf the sensing=element.

" hydrophone.

The invention described herein may bem'anufactured and used by or for the Government of the United States of America without the paymento f any royalties thereon or therefor.

The present inventionrelates to hydrophone calibration v apparatus and morepartiicularly to'a' 'calibrator for verifyhydrophone, I v

The detection and measurement of noise in'water is accomplished by the use of a highly sensitive, submerged ing' the status of calibration of apreviously calibrated hydrophone which is remotely located from'j other associated equipment comprising the sound detecting and ts?! Stat s. fiflm 1 e r 2,918,651 i iatented Dec. 22, 1959 tained must be discarded as being wholly in error. Consequently, elaborate precautions have been resorted to in deaerating the liquidand the tank itself when the tank is initially filled. A common method of deaerating comprises the use of vacuum pumps for removing air from the liquid, and maintaining the tank under vacuum while the tank'is being filledwith the liquid.

The present invention eliminates the above disadvan-,

tages by substituting a gas for the liquid in the tank used in the prior art recalibrating apparatus. Although the purpose of the recalibration is to check the sensitivity of a hydrophone immersed in water, we have found that this recalibration can be performed with improved accuracy and convenience in a calibration chamber filled with gas. Further, a driver unit-has been modified for use with the gas filled chamberin order to achieve optimum results in the measuring process. The principal results flowing from the use of this novel recalibration equipment is that the apparatus is more reliable since the normal error always introduced by the presence of minute air bubbles in the water have been completely eliminated. The recali- ,...brating process can be made much faster, and by virtue measuring system. The submerged hydrophoneiis incontactwith water andis therefore subjected to" hydrostatic pressure,;sound pressure vibration and possible impact by objects in the waterr The sensing element for the hydrophone is enclosed within a casing which must .be acoustically transparent, that is, the' acoustic impedance of the casing must approximate the acoustic impedance of wa- I ter, andas a result, the casing isgenerally made of synthetic rubber.

The rubber casing,{ however,--fails under certain'adverse operating conditions thereby -=affecting the Both'tlie sensing element 'and'th'e mechanical support therefor are-of delicate constructionI-and as the hydrophone is used during nor- I 'fmal; o peration of the system, the sensitivity of the appa- 'ratus' changes thereby requiring its replacement. In the event the calibration of 'the'hydrophone-cannot be veriv fled periodically, any datacollecte'd from use of'the hydrophone must'be carefully scrutinized for possible errors. *Theinitia] calibrationand-recalibrationjof a hydrophone in accordance with the prior art, is an elaborate procedure which few. laboratories are equipped 'to perform; Furthermore, considerable time and expense 'are involved in effecting suchcalibration,particularly since the hydrophone must beremoved. and transportedto .a properly equipped laboratory fr'om a remotepoint.

One known method in the prior art-.of recalibrating a "hydrophone comprises placing. thehydrophone in a chamber of liquid either water or oil, andin-sealing the end of the ch'amber with afwaterproof joint. The side of the, chamber'is, provided with a platethat'is-driven by atransducer, and output leads are connected to the exposed end of the hydrophone which lead to appropriate recording equipment for measuringthe electrical voltage output of the. hydrophone.- With the'hydrophone installed as described -the transducerflis energizedfor creating sound pressure in the liquid which causes a voltage to be gen- .erated in 'the hydrophonerQThe voltage is transmitted through theoutput' leads to the recording equipment thereby giving apositive indication of the sensitivity of the There are numerous disadvantages t6 this prior art method] and apparatus for r'ecalibratin'g. a hydrophone.

The chamber must be provided with thick walls and special type sealing joints must be used for achieving a fluid- .quick and reliable method and apparatus for determining the sensitivity of a hydrophone.

It is another object of the present invention to provide portable ap'paratus for verifying the calibration of hydrophones under field conditions.

Afurther object of the invention is the provision of a chamb'erjhaving a gas serving as a conducting medium for a soun'df'pressure for energizing a hydrophone during a recalibration process.

A still further object of the invention is a method of comparison calibration wherein a hydrophone of unknown sensitivity can be evaluated when compared with a hydrophone of known calibration.

"Other objects and manyof the attendant advantages of this'invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. l is a diagrammatical'showing of a calibrator including associated equipment .for determining the sensitivity of a hydrophone; and- Fig. 2 is an axial cross-sectional view of a calibrator of the'present invention showing details of the sealing joint and the driver unit associated therewith.

Referring now to the drawings wherein the same reference character designates like or corresponding parts throughout the several views, there is shown in Fig. 1 a calibrator 10 comprising a chamber or casing 11 having a transducer 12 affixed' in the side thereof for creating sound pressure in the tank. A hydrophone 14, whose sensitivity is being checked, is fitted within the tank 11 and iis connected to an amplifier 16 at its other end by means of cable 17 When the hydrophone 14 is excited by the sound pressure generated by transducer 12, theelectrical output of the hydrophone is amplified in the amplifier 16' and impressed on a recorder 18 that is preferably mechanically or synchro-coupled to an audio oscillator 20. The output of oscillator 20 is connected to th'e'transducer 12 by means of conductors 22 for providing the transducer 12 with an electrical voltage hydrophone 14.-

3 Referring more specifically to the parts, the hydrophone 14 comprises an outer tubular casing 30, Figure 2, having at one end, one or more pressure responsive elements 32, such as piezo-electric crystals. A jacket of rubber coversthis end of the hydrophon'e fdrcoupling it to the medium within the tank, whether it be a liquid or gas. I p t M In accordance with the. invention, the calibrator 10. is provided in which low frequency sound pressures are created by the transducer which afiect the sensitive element of the hydrophone for determining its sensitivity. The calibrator comprises the chamber 11 for receiving hydrophone 14 and a cavity 34 is formedbetween the walls of the chamber and the outer surface of the hydrophone. The cavity 34 provides an air space around the enclosed part of the hydrophone but the inside dimenstylus for recording a graph of sions of the cavity are restricted to a value assmall as is practicable, since the high frequency response is limited by the longest dimension inside the cavity ,which deter mines the first resonance. It will be evident that only one chamber or calibrator is necessary when hydrophones of the same type are being tested since those in the same type are all of the same dimensions.

In order to properly position the hydrophone 14with in the chamber, a ring 36 is afiixed adjacent the inside] bottom end of the chamber and is held thereto by means of screws 38, or other appropriate securing means,

provision of ring 36 serves to correctly positionjthe hy-f drophone vertically and coaxially of the chamber, and a water space 40 provided in the bottom serves to]col-' lect and contain any water dripping from the hydrophone when it is inserted when wet. A further advantage of the ring is that it decreases the length of .the column of air which directly reduces thev'olume. of 'air that must be driven by the transducer,therebyjobtainiug a the response ofthe hydrophone with respect to the frequency of a driving audio signal.

The audio oscillator 20 is provided with a jack 80 for reception of leads froma voltmeter 82. The oscillator is further provided witlrtreqtiency adjustment means 84 and voltage adjustmentmeansagas iscustomary in this type apparatus. Thechartmovingz means of recorder 18 is preferably mechanically or synchro-coupled to the frequency adjustment means' 84 of theioscillator 20, such coupling being indicated thebrok en lines :88 in .Fig. 1 extending between the escillator and the recorder.

As above-described, the driver, lzflismconnected to. the

. variable frequency audio oscillator 20, the output voltage of which is monitored by the voltmeter 82. When the electromagnets ,66 a1 e energized by the signal from the oscillator, the diaphragm 68 is actuated thereby exz citing the volume of air in the cavity The magnitude of the voltageimpressed onthe driver unit isadjustable For a given frequency and, voltage across the drive and a fixed volume of air the cavity, a sound is created in the calibrator, the absolute magnitudefof which need not be knowni Thejhydrophone outpu t then In operatiom the hydrophone 14 whose calibration is known by ,a prior free field calibratidr is, positioned in cclianibe r' 11 and the seal elosed thetop end thereof...

Appropriate connections are theurriadepetweenariend of both the hydrophone and the inner peripheral surface of the chamber and is held in place by means of screws 44. A step 46 is formed on the inner portion of sleeve 42 for receiving packing glands 48 which are compressed by a knurled packing ring. SZ-acting .on threads 54011 the sleeve, thereby providing an efiective seal for en-' closing the lower part ofthe hydrophone within chamberll. t r W The driver 12, attached to a sideof thechamber 11, is a transducer for converting an alternating current or voltage into sonic pressure, and may be of anydesired type, such as a dynamic unit or an electro-ma'gnetic 'unit having a diaphragm, as shown. is provided in a side ofthe chamber 11 and is enclosed A circular opening. .56

the l' ydrophone andv the. amplifierprecorder, oscillator,

and the driver unit '12. A predetermined frequency and voltage is then selected onthefrequency ,andyoltag e adthe oscillatorw ill be of the samefrequency asthat which This signal is th en applied t a sound pressure-in the amplified bythe amplifier 16 and recorded on r ecorder i8. The hydrop hone is then put into use Eitherperi odically or whenever the calibratibmpfthe hydrophone is suspect, the hydrophone is recalibrated by repeating the, flabove steps; lf the readings taken again in the tank are the same, the original free-field calibration is, still valid.

However, if the value is different frorr lj thatoriginally observed, then; the, sensitivity \,.ofj.gthe hydrophone has, changed, the amount of, change being determined byjthe,

I differences between the initial .readings, and, the. reading by a tubular neck 58 attached thereto by any appropriate securing means, such as welding. The tubular. neck 58 is provided witha flange 60 which spaced from but connected with a mounting plate 62"b'y.a plurality of screws 64. The operating elements of the driver unit are positioned between the mounting plate and flange. 60, the elements comprising a receiver similar to a telephone receiver unit.- Electro-magnets 66 on plate 62, move a solid diaphragm 68 sealed to anouter casing 70 of the driver unit. An airtight chamber is provided for the driver unit by the insertion of a gasket 72 having an opening 74 therein, which is compressed between the open end of tubular neck 58 and the casing 70;

The amplifier 16 is of a common type andi sp rovid e d with a cathode-follower stage to provide a low-imped--- ance source for the cable 17. The recorder 18 is'of a type having a movable chart and a laterally movable obtained ,under this test. The abovelprocess is, then repeated for varying voltages and frequenbies and-,the re sults obtained compared with that' originally madeion the a hydrophone. Since therecorder 18is ganged with the frequency. control, a'graph is drawn,:which graph'fiserves to correlate the amplitude outputoffthehydrophone the frequency'of 'the oscillator.

auxiliary or secondary purpose "in 'that thegsen sitivityoty an unknown hydrophonecan be measured when a hydro-d phone of a known calibration of the: same class, fiis-available for useasa st'andard. This can be accomplishedlby facturer or at "a jtesti ng facility, andknowing its calibra thepressure in ft r phone of unknown calibration, but 0t the samemake'and comparing the output voltages of the know'n an'df unknown hydro'phones, the sensitivity of itlielat'ter canfbe,

are the same, the sensitivity-of the the am -Y lit e Q tput o th se d un pw r phone is half that of the hydrophone of known calibration, then the second hydrophone is one-half as sensitive.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A calibrator for use with a cylindrical hydrophone adapted for immersion in a liquid comprising a cylindrical housing having an open end, means at the upper end of said cylinder for sealing said hydrophone therein thereby forming an air cavity within said housing, a tubular neck sealed to a side of said cylindrical housing in communication with said air cavity, an electromagnetic driver sealed to said neck, said driver being adapted to produce sound pressures in the cavity, a source of alternating current to activate said driver, said source having means to provide an output of a constant predetermined magnitude and adjusting means to adjust the frequency of said output, and recorder means connected to the output of the hydrophone to record the magnitudeof the response thereof to the sound pressures for determining the sensitivity of said hydrophone over a range of frequencies.

2. The combination according to claim 1 including a mechanical drive connection between said recorder and said adjusting means, thereby synchronizing the recorder with the adjusting means.

References Cited in the file of this patent UNITED STATES PATENTS 1,536,763 Brown May 5, 1925 2,465,468 Mott Mar. 29, 1949 2,530,383 Estes Nov. 21, 1950 OTHER REFERENCES 20 p. 59 relied upon. 

